The moratorium on commercial whaling, one of the world's major environmental achievements, is in danger of being abandoned after 24 years at a meeting of the International Whaling Commission (IWC) which begins this week in Morocco.

A proposed new deal, which stands a realistic chance of being passed at the conference in Agadir, would allow the three countries which have continued killing the great whales in defiance of the ban – Japan, Norway and Iceland – to recommence whaling legally in return for bringing down their catches.

However, many conservationists do not believe that catches will actually fall under the proposed new agreement, and one of the world's leading whaling scientists recently described it in testimony to the US Congress as "a scam ... likely to fool many people".

Yet the chances of the deal going through are increased by a bizarre bureaucratic twist which may mean that European countries such as Britain, which are opposed, may not be able to vote against it in the final section of the meeting, which begins in three weeks' time.

"This is a great deal for the whaling countries," said Mark Simmonds, international head of science for the Whale and Dolphin Conservation Society. "In Norway they're already celebrating. But it's potentially a tragedy for the whales."

Should the moratorium be dismantled, it would represent one of the most damaging setbacks ever for wildlife conservation. The ban, which was agreed in 1982 and became operational in 1986, was introduced after a long and intense campaign by environmental pressure groups such as Greenpeace.

They were protesting against the intense cruelty of whaling, where the killing is done by firing explosive harpoons into the large, intelligent animals, and also against the fact that many of the stocks of the great whales had been drastically reduced by over-hunting, with blue whales driven to the brink of extinction.

Although large-scale whaling came to an end with the ban, and populations began to recover, three countries carried on killing: Japan, by labelling its hunting "scientific research", and the Norwegians and Icelanders by lodging formal objections. Since 1986 the three nations have between them killed more than 30,000 whales, the Japanese leading with more than 1,000 whales a year – mainly minke whales, but also Bryde's, fin, sei and sperm whales.

But the global total of kills has nevertheless fallen to a tiny fraction of what it was, and the moratorium has been an unqualified success from a whale conservation point of view.

The deal which may do away with it, which has been on the table for three years, was first thought to be merely a diplomatic compromise to end the perpetual confrontation at IWC meetings between the whaling nations and the anti-whaling countries. But recently it has become clear that it had a different purpose, and was cooked up in the US – by leading figures in the Bush administration, among them being Senator Ted Stevens of Alaska, who, until his conviction for taking unreported gifts in 2008, was the longest-serving Republican senator in American history.

One of the most powerful figures in US politics, Senator Stevens sought a deal with Japan after the Japanese caused problems for the US by objecting (as a bargaining counter in IWC negotiations) to the whale-hunting quota for Alaskan Inuit peoples, who have a traditional hunt for about 50 bowhead whales.

Senator Stevens is believed to have put pressure on the then-US Whaling Commissioner and IWC chairman, William Hogarth – whose budget, in the US National Marine Fisheries Service, Mr Stevens controlled as a member of the Senate Appropriations Committee – to open talks with Japan, which Mr Hogarth duly did at the 2007 IWC meeting in Anchorage, Alaska.

Mr Hogarth's proposals, which would have allowed the Japanese and others to restart whaling commercially, were eventually thrown out by the IWC. Yet the deal now back on the table is essentially a modified version of his original plan, which is even more favourable to the whaling states.

It is notable that the US, which used to have to negotiate its Inuit bowhead quota every five years, will get a 10-year quota if the new deal goes ahead.

Not knowing the background, many environmental campaigners have been baffled by the fact that the US, which for decades had been one of whaling's staunchest opponents, seemed to be leading moves to end the moratorium. Now they understand why.

However, although Mr Hogarth's successor as US Whaling Commissioner, Monica Medina, has for months also been pushing the deal hard, President Obama has begun to take an interest in the issue and is understood to have expressed disquiet, and Ms Medina's attitude has shifted.

At a briefing late last week she said that the US could not accept the deal in its present form, but was interested in a new agreement. A close observer of the situation in Washington said: "The US position is now in flux."

Britain's position is clear: the UK is against the deal. "The UK opposes all forms of whaling other than limited whaling by indigenous people for clearly defined subsistence purposes," said the minister for the Marine Environment, Richard Benyon.

"We have a number of concerns with the IWC proposal, not least that this would effectively end the international moratorium on commercial whaling that the UK has fully supported. We will be encouraging our partners in Europe to vote against the proposal, and hope that the EU will show leadership in this important global conservation measure."

But therein lies a difficulty. European Commission lawyers have said that the EU must vote by consensus in the IWC, and if consensus cannot be achieved, the 25 EU member states – who form a substantial voting block in the 88-member IWC – must abstain.

Most EU nations will be against the deal, but it is possible that Denmark, with links to indigenous whaling communities in the Faroes and Greenland, might vote in favour, in which case Britain and other opponents would not be able to cast their votes against it, and the chances of the deal going through would soar.

The issue is still being negotiated between European Commission lawyers and member states, including Britain.

How the new hunting quotas will work

The idea of the new deal is to bring all whale killing back under the aegis of the IWC, so the Japanese would cease the fiction of "scientific whaling" and the Norwegians and the Icelanders would come back into the fold. But their hunting would have to be legitimised – they would be given official "quotas" to kill, for the first time in 24 years.

In return, say proponents, these quotas would be capped at a lower level than the killing currently going on, so, at the end of a 10-year period, fewer whales would have been killed. In addition, there would be other conservation measures, including observers on boats, a sanctuary in the South Atlantic, and a DNA database to trace the origin of whalemeat.

The opponents say that legitimising the commercial hunting of whales would open the way to a free-for-all. But, even more, there is no guarantee that the capped quotas would be safe, adequate, or even respected.

One of the world's leading experts on whaling, the British biologist Justin Cooke, who is the representative of the International Union for the Conservation of Nature on the IWC Scientific Committee, took the deal apart in the US Congress, in evidence to the House Committee on Foreign Affairs. Dr Cooke said: "The proposal is disingenuous and I suspect that it will fool many people." It was a scam, he said, in which the calculation of how many whales could be killed was being left to politicians rather than scientists.